1. Field of the Invention
This invention relates to a new process for the production of NCO prepolymers based on selected starting materials having an NCO functionality of greater than 2.1 and a monomer content of less than 2% by weight, to the NCO prepolymers obtained by this process and to their use in polyurethane lacquers and coatings.
2. Description of the Prior Art
The production of isocyanate-terminated prepolymers is known and described, for example, in DE-OS Nos. 1,595,273, 2,845,514 and 3,401,129: in EP-A-No. 107,014 and EP-A-No. 118,065: and in U.S. Pat. Nos. 3,706,710, 4,184,005, 4,195,009, 4,211,804, 4,273,912, 4,282,123 and 4,385,171.
In many cases, it is a disadvantage that the prepolymers obtained have relatively high contents of monomeric diisocyanates. Prepolymers such as these are unsuitable for many applications because the high monomer contents make safe further processing by the user impossible for toxicological and industrial hygiene reasons. In addition, excessively high monomer contents frequently cause problems through the resulting excessive reactivities of the prepolymers with their reactants and, thus, overly short pot lives and processing times.
Accordingly, there have been no shortages of attempts to produce low-monomer isocyanate prepolymers and numerous processes have been developed to that end. Thus, on completion of the NCO/OH reaction, monomeric diisocyanates may be separated from the prepolymers by thin-layer distillation (for example DE-OS No. 1,595,273) or by stripping distillation using a relatively high-boiling auxiliary solvent (for example U.S. Pat. No. 4,385,171), providing the diisocyanates in question are volatile. In addition, the production of low-monomer prepolymers by heterogeneous reaction (DD-PS No. 155,323) or by a two-stage reaction using different diisocyanates (DE-OS No. 3,401,129, EP-A-No. 107,014, EP-A-No. 118,065) has already been described.
However, all the state-of-the-art processes mentioned above for the production of low-monomer isocyanate prepolymers are attended by the serious disadvantage that, in general, they involve either an elaborate multistage reaction technique or an additional step in the form of distillation on completion of the production process.
In addition, it has been found that certain isocyanate prepolymers required to show special properties cannot be produced by the processes mentioned above if, as a liquid or a melt, they cannot be freed from monomeric diisocyanates by thin-layer distillation, for example on account of their consistency. This group includes, for example, prepolymers based on aliphatic diisocyanates and branched polyester polyols which, accordingly, cannot be obtained in low-monomer form despite sufficiently high isocyanate contents.
Two-stage reactions frequently give prepolymers of relatively high molecular weight and, hence, relatively high viscosity. However, these prepolymers are not suitable for high-solid systems because these systems require low molecular weight isocyanate prepolymers which can be produced in concentrated, moderately viscous solutions despite their high isocyanate group content.
Accordingly, an object of the present invention is to provide a new process for the production of prepolymers containing free isocyanate groups which has the following advantages: The process is intended to enable low-monomer NCO prepolymers (content of free starting isocyanates less than 2 and preferably less than 1% by weight) to be produced in a single stage without subsequent removal of excess monomers. The NCO prepolymers are intended on the one hand to have a comparatively high NCO content (4 to 12% by weight) and an (average) NCO functionality of greater than 2.1, most preferably of at least 2.5 and, on the other hand, to contain incorporated, relatively high molecular weight polyester segments as elasticizing structural units, so that coatings produced from the prepolymers satisfy stringent practical requirements with regard to the mechanical properties of the lacquer coatings.
The object as stated above may be achieved by the provision of the process according to the invention which is described in detail hereinafter. In the process according to the invention, the starting diisocyanates used for the production of the prepolymers are organic diisocyanates containing an isocyanate group attached to a primary aliphatic carbon atom and an isocyanate group attached to a tertiary aliphatic or cycloaliphatic carbon atom in combination with selected polyhydroxyl compounds. Although it was known from DE-OS No. 3,402,623 (EP-A-No. 153,561 or U.S. Pat. No. 4,613,685, herein incorporated by reference in its entirety) that aliphatic-cycloaliphatic diisocyanates of this type are eminently suitable for the production of NCO prepolymers, it was not obvious from the disclosure of this publication that the object which the invention seeks to achieve, as stated above, could be achieved using diisocyanates of this type in combination with selected polyhydroxyl compounds of the type described in more detail hereinafter in certain quantitative ratios.